Discovery 2. Manual - part 739
BODY CONTROL UNIT
DESCRIPTION AND OPERATION 86-3-23
The BCU checks the status of the following inputs to determine when to provide the rear window enable and sunroof
enable output signals:
l
Ignition state.
l
Driver's door open.
l
Passenger or rear door open.
The rear window enable and sunroof enable lines are on when the ignition switch is in position II, and are switched
off if the ignition switch is moved to the crank position.
The BCU monitors the circuit current draw when the windows are operating to detect the occurrence of a stall
condition. If the BCU determines that the window has stalled, it will cut the power supply to stop the operation of the
window motor. The BCU only starts to check for a stall condition after the window has been operating for 500 ms. This
delay period enables the initial current surge to pass and eliminates false detections of window stall. A stall condition
is activated when a nominal current value is exceeded for a period greater than 300 ms.
If the BCU detects that it has been operating the same window motor in one direction for more than 10
±
0.2 seconds,
without the BCU detecting a stall condition or the control switch being released, it will automatically stop operation of
the window motor (timed out). Window operation can resume when the window switch is released and pressed again.
If the window was operating in one touch mode, the window operation will resume on the next press of the window
switch.
One touch mode automatically lowers the window to its fully open position, without the need for the window switch to
be kept pressed for the duration of the window travel. One touch mode is initialised if the window switch is pressed
for a period of less than 0.4
±
0.2 second. One touch mode continues to lower the window until the BCU detects:
l
A stall condition exists (window has reached bottom of travel).
l
Window switch is repressed in either direction for more than 0.4
±
0.2 second.
l
The BCU has operated the motor for a period greater than 10
±
0.2 seconds.
A timeout function is included which allows the windows and sunroofs to be operated for a period of 44
±
1 seconds
after the ignition key has been removed from the ignition switch. The timeout function will be cancelled if the BCU
detects the following conditions:
l
The driver's door has been closed after the ignition has been switched off.
l
Any door open after the ignition is switched off.
The preconditions are programmed into the BCU dependent on market specification.
The BCU does not directly control rear window operation, but it can request the IDM to remove the power feed to the
rear windows. The IDM enables power supply to be provided to the rear lift relay in the passenger fuse box. When the
relay coil is energised the relay contacts close and power is applied to the rear window motor. If the window lift
isolation switch located in the centre console is latched out, the earth path from the LH and RH rear door window
switches is broken and operation of the rear window motors through the use of the rear window switches is prevented.
Rear window operation using the console switches is not affected by the isolation switch.
In the event of a communications link failure occurring while rear window enable is on, the relay remains on until the
ignition switch is turned to the off position.